Minimum-voltage network and system



Jan. 15, 1946. HAR ER 2,393,043

MINIMUM-VOLTAGE NETWORK AND SYSTEM Fild Oct. 30, 1942 INVENTOR T4 7:; (gm/Jr L/Varaer:

Patented Jan. 15, 1946 MINIMUM: VOLTAGE NETWORK AND SYSTEM" Edwin L. Ha'IdBI' FOI'CSt Hills, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of'Penn'syl'vania -ilrpplication October 30, 19,42, SerialNo. 4 6 3,973

24 Claims.

, My invention relates to voltage-responsive; dervices whichare designed to respondv to thesmallest of two or more volta es; andlaiso to polyphase lines which are subject tg single-phase;faults. In one of its aspects, my invention relates to apparatus for responding v 'towhichever phase of a polyphase electrical, quantity islowest in magnitude. Another, aspect of invention relates to a minimum-voltage network for obtaining a smallest-voltage response,

I Heretofore, whenever, a low-voltage-response has been required, in a polyphasesystem, it has been obtained either in a single-phase-responsive device, or in a polyphase-r'esponsive device, or in a single-phase element energized from a phasesequence selective netwo rk. In pclyphase systerns or devices in which a. single-phase voltageresponse was not acceptable, because of the possibility of a single-phase fault occurring onany This is true, for eiiam'ple, in quick-response ex-' citation-systems, which have been known since 1927, as shown in the Evanset a1. Patent 1,692,- 495, wherein it was. shown to be desirable to quickly increase the excitation of a synchronous generator or condensenat times of a single-phase fault on a polyphase system. It is not satisfactory to excite the voltagesregulator from a single phase of, the line-voltage, because, if this were done a single-phase fault on someother phase might, under some circumstances, produce a mo,- mentary-voltage-rise on one of theunfaulted phases, which would cause the quick-response excitation-system to start to rapidly decrease the excitation, when an increase was actually required. This difiicultyhasbeen overcome by resort to a, polyphaserresponsive voltage-regulator. Similarly, in the various single-element protective relays, which have-been known heretofore, for. polyphese=l nes,;where it has been necessary tg-utilize voltage-restraint, or a voltage-response of: any: sort, it haslikewisebeen necessary to. resort tethe polyphasef voltage-response, in order t -se ure us ful w s- ,In, accordance with, my present invention, I utilize aminimum-voltage response, that is, a response to the smallest of two or more independently varying voltages, or other electrical quantities, and specifically, inthe caseotthreephasesystems; a response to thesmallest ofthe three line-voltages, either line-to-line voltages, or line-to-neutral voltages, or some other polyphase: electrical quantity of the line.

An object o finy invention, therefore, is to pmvide a Dolyphase system having a minimum-voltage-responsive device, which may be eithera voltage-regulator or the restraint-part ofanimpedance relay, directional relay, or differential relay, or an undervoltage relay, or some other protective relaying device, either mechanical or tube-type.

A further object of my invention is to provide a. novel minimum-voltage network, in which the terminals of a voltage-responsive device are connected to two or more unidirectional-voltage sources, through serially connected rectifiers which enable the source havingthelowest volt age to freely draw off enoughcurrent from the terminals of the voltage-responsive device, so: as to'bring down the voltage of the voltage-responsive device to a value closely approaching the lowest controlevoltage orconnected source, whereas the reverse-current blocking-effect of the serially connected rectifiers prevents substantial current-flow in the, reverse-direction, when the connected control-voltage-is higher than the voltage appearing across the terminals of the voltage-responsive device.

With the foregoing and other objectsv in View my invention relates to the structures, apparatus, combinations, systems and methods hereinafter described and claimed, and illustrated in the accompanying drawing, wherein: p I

Figure 1 is a diagrammatic view of circuits and apparatus illustrating my invention a being, applied to a minimum-voltage-responsive device in the form of a voltage-regulator for a quick-response excitation-system for a polyphase synchronous machine,

Fig. 2 is an equivalent-circuit, diagramwhich will be referred to in the mathematical analysis of the minimumvoltage selecting network shown in Fig. l,

Fig. 3 is a diagrammatic view of: circuits and apparatus illustrating my invention as applied to a single-element relaying system for a polyphase line, utilizing a. single impedance relay which is actuated in response to the largest phasecurrent of the line, and which is restrained in response to the smallest phase-voltage of the line, Fig. 4 is a similar view, except that the relay is actuated in response to a phase-sequence quantity derived from the polyphase line-voltage, Fig. 5 is a diagrammatic view of circuits and apparatus illustrating the application of my invention to a relaying system in which a comparison is made between the minimum voltage appearing in any phase of one polyphase bus and the minimmn voltage appearing in any phase of another polyphase bus, and

Fig. 6 is a diagrammatic view of circuits and apparatus illustrating the use of my invention in connection with an under-voltage fault-detecting relay, and also illustrating the utilization of a tube-type of relay, as distinguished from a mechanical-type relay.

In Fig. l, I have illustrated my invention in connection with a three-phase synchronous dynamo-electric machine In, having an excitation-system represented by the slip-rings i i, the voltage-regulator l2, and the source of excitingcurrent energy I3. The primary windings of the synchronous machine I are connected to a three-phase line I4.

In accordance with my invention, as shown in Fig. l, I provide a minimum-voltage-selective network i5, for responding to the smallest of the three line-voltages, which are derived by means of three potential-transformers Ta, Tb, Tc. The three derived line-voltages are rectified, through rectifier-bridges Ba, Bb and Be, the outputs of which are supplied to three rectified-current resistors Rr, Br and Br, so as to produce, in the several resistors Rr, a unidirectional voltage-drop proportional to the magnitude of the corresponding phase-voltage of the line H. Ordinarily, some sort of voltage-smoothing means is desirable, for smoothing out the ripples in the rectified current, and to this end I have illustrated filtercapacitors Ca, Cb and Co, shunting the respective resistors Br, and intended to be symbolic of any desired ripple-suppressing means, a number of which are known to the art.

My minimum-voltage network I is also provided with a pair of output-terminals m, n which are the terminals of a relay R or other voltageresponsive device which is intended to be responsive to the smallest of the three control-voltages appearing across the three rectified-current resistors Rr. The output-terminals m, n of the network IS, in Fig. l, are in the general case, also energized from an auxiliary unidirectional-current source of poor voltage-regulation, by which is meant a source of unidirectional voltage which decreases with increasing current, which is represented, in Fig. 1, by a battery or other constantvoltage source Es having a serially connected resistance Rs, or other impedance, either constant, or non-linearly responsive to the currentflow. The three line-voltage-controlled resistors Rr are severally connected, in as many parallelconnected branch-circuits, across the output ter-- minals m, n of the network IS, in the same polarity as the auxiliary source Es, that is, with the positive terminals of all of the sources connected to the network-terminal m, for example, except that, in the case of the three control-voltage resistors Ptr, three serially connected rectifiers IS, IT and i8, such as small contact-type rectifiers, or other asymmetrically conducting circuit-means, are respectively interposed in the circuit-connections, one in each, so as to provide a relatively free or good-conducting path for current flowing from the network-terminals m, 7L to any one of the three control-voltage resistors Ry, but interposing a relatively high resistance, or even substantially an open-circuit effect, for preventing current from freely flowing in the reverse direction.

In the particular system shown in Fig. l, the

relay R is the voltage-coil of the voltage-regulator l2, which is utilized to control, in some manner, the vibrating-contacts IQ of the regulator.

The operation of the minimum-voltage net work l5 of Fig. 1 will best be understood by reference to the equivalent-circuit diagram of Fig. 2, in which the serially connected rectifiers i6, i1 and I8 have been replaced by their equivalent resistances Rm, Rb and Re, respectively, having a small resistance-value F in the forward direction, and a high resistance-value NF in the reverse-direction, as indicated by the arrows, N being the rectification-ratio. The relay R is represented, in Fig. 2, by its resistance R. The voltages appearing across the three control'voltage resistances Rr are designated Ea, Eb, EC, respectively, with the understanding that the subscript a is applied to whichever one of three voltages happens to be the lowest, so that, if these three voltages Ea, Eb, and E0 are not all equal, it is always assumed that Ea designates the lowest of the three control-voltages.

In Fig. 2, the network-voltage is ll b n l scientific az sz a a s As long as Ea, which is the designation applied to the smallest of the three controlling-voltages. is less than Emu,

Since F is small as compared to R, F will draw enough current to bring down Emn to a value close to Ea, so that Emn will be less than either one of the two larger controlling-voltages Eb or E0, so that Equation 1 thus becomes In order for Ea to be less than Emu. it must be less than the voltage E0 which the network would have had without the Ea-branch. Thus This imposes an upper limit on El, above which the network-voltage Emn will not be responsive to EB. This upper limit of E. may be designated,

mix

aaasme tram said? transformer 13;, so: as-v todetemine Thus:

I I|i 1fa (7) whence? E E E; rm

Substituting from ,(4)- and solving,

v 1 ,-;-.(E.+ we El) 7 u- Ft F 2- F F" rfirWX RIFE V This imposesalower. limitoniEa, below which thanetworkevoltage Em will not. be controlled by Ea. Thislower limit of Ea may be designated,

Dividing: (6) by 010), and rearranging the term The ratio of themeasured voltage Emnto the controlevoltage Ea isfound; from (4) andv (6);

Therelay-response-Emn is proportional to the control-voltage Ea within av certain. accuracy which we may designate asiP. percent, over the range from Ed Eflmr to E=Emm A low percent,- age of error, P, is desired, both in relays which are required to respond accurately tO Es. over a considerable range, Emaxyto Emin, and also inrundervoltage or. dropout relays in which the rate of change; in the relay-voltage Emn should be'as fast. as possible, in comparison to the rate of change of the controlling-voltage Ea a E E 2 1+ min' Substituting; fromllb and solving for; P;

P" F I?" F} F en m ation) nc) The fraotioniF/Rr will-inherentlybe small; and hence as shown-byEquation 1.4,theerror P. will be small;v This is so, because the forward-resistance F of the rectifier l6 which is interposed between the network-voltage Emu and the control-voltage Ea will naturally be made as small as practicable, while the bridge-connected resistance R: will have to be made large enough to avoid imposing an excessive volt-ampere burden Wr on the potential-transformer Ta. When Ea=Eb=Ec, or Ea=(Eb+Ec) /Z, this potentialtransformer burden Wr Will be approximately equalto,

Since: F/Rr is thus small, Equation Iii-shows that-the spread bBtWBGHEmIXZ andfEmmis depend ent; mainly upon H,. I 2g 1 RT NF Obviously; the smaller. the spread between Em: and Emin, in Equation 11, the smaller may be the relay-resistance R in comparison" to the bridgeconnectedresistance R thusincreasing the relay-energy, which is,

2711 W-- I p (16) From theEquation 11, it'is very evident that, tor a given ratio ot Emsxto Earn, over which-the relay is to be responsive, and for a mechanical relay, which usually needs: a maximum energyinput vWiuto the relay' at any given relay-voltage Emm, (as distinguished from a tube-type relay); it is necessary for the relayeresistance R to be as small as possible, incomparison to the bridgeconnect ed resistance Re, which can be accomplished only; making 2R/NF and R/Ri as smail' as possible in Equation- 11. This means makingN'andRs as large as possible, as will'now be discussed;

Tomake Nilarge, meansto choosea good rectifler-for the serially-connected rectifier l6 which is interposed between the network-voltage Emu and the control-voltage Ea.

To make Rs as-large as possible, it could be made infinite which is" to say that the source Es could-beomitted; except for the requirement as to the maximum control-voltage Emax to which the relay must respond. With Rs infinite; Equation 6 showsthat Emax cannot belarger-than (Ez +-E)/(2+NF/R); and the higher the rectification-ratio N; the smaller will be the fraction 1/, (l-l-NF/ 2R) For-any relay which is'required to begin to respond to; the smallest control -voltage Ea before E's drops to a value as'low as (Eb+Ec) (2+NF/R), it is necessary to utilize the source Es; and hence; for serving such a relay, Rs cannotbe made infinite;

For a network in which the source Es is required, in order to, make the network responsive toasufli'ciently high maximum-value Emax of. the control-voltage Ea, the network-constants may advantageously be selected,. if, desired, so, to obtainav maximum relay-energy W at any desiredzrelay-voltage Emn. If; it is desired, for, ex-. ample, to make the-relay-energy W as large as possible when the control-voltage Ea has its maximum value Emax at which E. will exercise any control over the relay-voltage Emn, the relay energy W under these conditions may be obtained by substituting from Equations 4 and 6 in Equation 16; and the conditions for a maximum relay-energy W may be obtained by put ting d W myielding,

1 NFR tri R NF This means that if the source E and its serially connected resistance Rs are used, and if the relay-energy W is to be as large as possible, the reverse-current resistance NE of the serially connected rectifiers I6, I! and I8 must be more than twice the relay-resistance R-perhaps at least four times as much, in any practical case.

Substituting from (17) in (11), we find that For the particular case in which it is required that the relay shall begin to respond to the smallest control-voltage Ea when Ea first begins to drop below Ea=Eb=Ec, Emax will be (Eb-l-Ec) /2, and Equation 19 reduces to 11. Eb+ E. 1+ )=(Eb+ E. 0)

Equations 17, 18 and 20 thus show the essential design-characteristics of a network designed to provide a maximum energy-input into the relay when the three control-voltages Ea, Eb, and E0 are all equal, the network being also designed so that it is able to respond to Ea when it first starts to drop below Eb or Be, that is, below (Eb-l -Ec) Where the potential transformers Ta, Tb and To are connected to the difierent phases of a constant-voltage line, which is the normal case, and where the network is not required to respond to the lowest control-voltage Ea until it drops to some value below its normal maximum value, which is the normal line-voltage, it is possible to utilize a I storage-battery for the auxiliary source E5, and to make the voltage of this storage-battery somewhat less than the normal linevoltage, or somewhat less than (Eb+Ec) /2. Under these circumstances, under normal line-voltage conditions, the three line-voltages will tricklecharge the battery Es through the back-currentfiow through the three serially connected rectifiers l6, l1 and [8 of Fig. 1, which are represented by the resistances Ra, Rb and Re in Fig. 2, thus saving the cost of a separate trickle-charging outfit for the battery. In other instances, a direct-current station-bus may be utilized for the auxiliary source Es.

In the operation of my invention as shown in Fig. 1, the regulator-coil R is thus impressed with a voltage which is responsive to the smallest of the three phases of the polyphase voltage 01 the three-phase line H, the manner of this response having fully been explained. Under normal linevoltage conditions, the vibrating regulator-contacts hold the voltage or the synchronous machine III at any desired value. If, now, a singlephase fault occurs on the line H to which the machine It! is connected, the line-voltage may not always immediately reduce on both or the sound phases, but it will be sure to be immediately reduced on the faulted phase, and my minimum-voltage network causes the regulator l2 to respond to the smallest voltage, which will be the voltage of the faulted phase, thus causing the regulator to promptly start the excitationresponse in the proper direction. Thus, when the regulator-contact I9 is closed, the excitation oi. the machine I!) is being quickly increased. In this manner, the benefits of quick-response excitation are obtained with a minimum-voltage response without necessitatin a special regulator which is responsive to a polyphase voltage.

In Fig. 3, I have illustrated my invention as applied to the protective relaying apparatus of a three-phase system which is represented by a three-phase bus 20 having a three-phase line 2| connected thereto through a circuit breaker 22. The circuit breaker 22 is provided with an auxiliary breaker-switch 22a, and a trip-coil TC. The relaying system shown in Fig. 3 is a singleelement relaying system utilizing an impedancerelay 23 which is illustrated as a polarized relay having an operating coil OC and a restraining coil RC. The relay-contacts of the impedancerelay 23 are utilized to energize the trip coil TC of the breaker through an auxiliary relayingcontact D, which may be the contact of a directional relay or other discriminatory relay for supervising the action of the impedance-relay.

The operating coil 00 of the impedance-relay 23 in Fig. 3 is energized from a bank of delta.- connected line-current transformers 24, which energize three auxiliary transformers 25, the secondary circuits of which are loaded with resistances 26, the voltage of which is rectified by means of half-wave rectifiers 21. The secondary windings of the auxiliary transformers 25 are provided with mid-taps 28 which are connected to one terminal of the operating coil OC, while the six rectifiers 2'! are connected to the other terminal 29 of the operating coil 00. In this manner, the operating coil 0C is energized in proportion to the largest phase of the three deltaphases of the line-current, in the manner described and claimed in my Patent 2,242,950, granted May 20, 1941.

The restraining coil RC of the impedance-relay 23 in Fig. 3 is energized from my novel minimumvoltage network IS, in the manner already described for Figs. 1 and 2. In Fig. 3, I have shown a trickle-charging means for the auxiliary battery Es, said trickle-charging means comprising an auxiliary potential-transformer T5, and a trickle-charging rectifier 30. In Fig. 3. instead of utilizing bridge-connected rectifiers Ba, etc., for deriving the line-voltage-responsive unidirectional control-voltages, I have shown, by way of illustration, a known type of double-half-wave rectifier-connection utilizing the midpoint of the transformer-secondary as one of the terminals n of the rectified-current output.

In the operation of the new type of impedancerelay 23 which is shown in Fig. 3, it will be observed that the operating ccil 0C is always energized in response to the line-current phase having the highest. magnitude, which will, of course,

he thednulted -phase, :in the: event of a single.- nhase fault. While the restraining coil RCis alwaysxenergized in response to the line-voltage phase havingthelowest magnitude, which means, of cours the phase on which the fault occurs.

Thuaa in line-toround fault on p as A will be-accornpaliiedby a fault-currentin the phase-A (or top) 'line-gconductor "2 I, causing this phasezoi the-line current to be'a maximum, and energizing the operatingjeoil C accordingly. At the same time, the delta voltages of the poten tial transformers To and Te will clip, by equal amounts, causing the restraining coil RC to be energized in accordance with-this voltage. The impedance relay :23 thus operates as if it were an alternatingcurrent relay havingan operating coil .00 connected to thephase-A line-current transformer 24, and having a restraining coil connected to either the phase-AB or the phase- Clidine-voltage transformer T; or Tc.

Or-a double-phase fault, onphases B and'C, for example, will be accompanied by a faultcurrent flowing in line-conductor phases -B and C, causingthe lineecunent in these phases to be a maximum and accordingly energizing the operating-coil'oC. At the same time, the delta BC 'voltaseiof the .potential transformer Tbwill be .a im m. accordingly energizing the restraining coil RC.

'Thus, my single relay 23 gives the same im- 1 dance-responseas vwouldhaverheen obtaine if three single-phase impedance-relays had been utilized, one for each-of the three phases, because mysineleqimpedance-relay 23 always operates as .if it were such a single-phase impedance-relay connected :in whichever phase happens to be the faulted phase. :It will be understood, of

course, sthfilt any desired impedance-relay iconne'ction's may be :utilized, using the word impatience :in its broad sense which includes reactance-responsive relays, and utilizing any of the-known orjdesiredicombinations of line-voltage and line currentshotibeingglimited to the use of delta current, :and Jn-phase delta voltage, as in the .particular-impedance-relay connections -illus- 1 trated in Fig.3.

In Fig. 4, I have-illustrated my invention as applied to. a-single-element relay-lngrsystem which is similar :to that which has already been described in connection with Fig. 3, except that? the impedance relayifl' hasits operating coil 00 energized in response to a ;selective-pha sesequence .current responsive filter-means which is designed :to drive ,a relaying-current which ;is

,responsiveto any one of a plurality of diflerent kinds oi faults on different phases of the line,

such as the filter which is described and claimed "in my "Patent 2,183,646, granted December :19, 1939. This current-network com- :prises a bank of star-connected line-current transformers 3 I, energizing a suitable network of resistances and mutual lmpedances, the volt:-

ageof .whichzis preferably transformed by means of an auxiliary transformer :32, which may, or

may not, be of a saturating type, to lirnit the maximum relaying current which is obtained under very severe fault-conditions. Since a polarized relay 23' is-illustrated in Fig. 4, {it is necessary to rectify theoutput of the'HCB-network, which is accomplished bybringingwut a mid-tap :33 :irom the secondary winding of the auxiliary 'transformer'fl and utilizing ;-two halfwave i land 85, as shown.

In theoperation of the system-1 shown 1 in :Fig. '4, the impedance element F 23' operatesthe same as a phase-sequence-responsive overcurrent relay, such as that shown in my Patent 2,183,646, with the addition of a minimum-voltage restraint, that is, with'restraint whichis responsive to the smallest of the three line-voltages, thus producing-an impedance-measuring efiect, and making therelay selectively responsive to the particular phase which is the most affected by the fault on the line.

Fig.5 illustrates the utilization of my invention to solvea diificulty which arose in connection with two-three-phasebuses BI and B2 which'were lo cated "at a station in which the buses were supplied with various terminals which were tied together, "in parallel-circuit arrangement, only through lon-gfeeders connected to a distributionnetworh'so-as to involve a certain amount of impedance in the paralleling connections'between the-buses. Thus, the bus BI is illustrated as being-energized'from asource SI which is connected to the three-phase bus-terminal Tl, while the bus 132 shown as being energized from a-source S2, connected to the three-phase bus-terminal T2. The bus-Bl isshown as supplying two three,- phase f'eeders T3 and T4, which extend out to the distribution-network while the bus B2 is shown as supplying twootherfeed'ers T5 and T6, which also extend out to the distribution-network. The bus-arrangement was a double-bus'doublebreaker scheme, however, so that'any one-of the six terminals T] to T6 might be connected to either cheer the two buses BI and B2, and-no current-transformers were available for usein difierential bussprotection, except the six banks of current-transformers connected in the several terminals'or lines TI to T16,-respectively. --By connecting together the corresponding phases of the six'banksof'current-transformers, it was ossible to differentially energize the differentialcurrent relays 1409, Min and Me, one for each 'phase,'so.-as to'respond to a. fault on one oi the two buses .-BI and B2, but-it was impossible, :by this-means, to discriminate as to which of the two-buses was faulted.

Under these circumstances, as shown in Fig. 5, I utilize two of my minimum-voltage networks lfi'and 1-5", one connected to eachof the'buses B1 and 132. For convenience of illustration, 1 have simplified the wiring diagram by connecting the minin'iu'mevoltage networks to the buseter- 'minals Ill and T2 respectively, but it will be under'stood that the connections would normally be made directly to the "respective abuses Bl and B2 themselves. The respective minimum-voltage networks 15' and 15", in :Fig. 5, eachselect the minimum, voltage on its own-bus, and these two :minimu'm voltages are compared by means of a dillf'rential relay 43' having twodiiferentially operating coils 44 i and 15, respectively, that the diiferential relay 33 drops down on, theside hav- 'i'ng the'minimum-volta'ge,'thusresponding to the bus which isiaulted, because the voltage of the faulted phase on the faulted buswill be lower than any'of the'phase voltag'es on the'sound bus.

The minimum-voltage responsive differential relay 43 of '5 re resents a broadlynew princip'l'e in relaying, in that a sifngl'esingle-phase currentor voltage-comparing -relay is utilized in connection with {two phase-selecting networks which are associatedwith two diflerent points between which a cliil'erentitil-rela'y operation is desired,-in a*poly-phase electrical system, th'e phaseselectingfnetworks being utilized to select the phase Which is distinctively different, and hence the phase'which is iminediately'affe'c'ted by a'single-phase fault, at its point in the system. Thus, if a differential current-responsive effect is desired. the phase having the maximum current may be selected, at each relaying point, by apparatus such as that shown at 29 in Fig. 3; and if a differential voltage-responsive effect is desired, the phase having the minimum voltage will be selected, at each relaying point, by apparatus such as is shown at l5, l5, l5" and l5', in the various figures. In any event, the distinctively different phases which are selected by the two networks are applied to a single differential relay, so that the single relay will automatically compare the fault-conditions at the two relaying-points, so as to select the point which is the most affected by the fault. This is a new relaying principle, and

I desire Fig. 5 to be understood as being illustrative of the same.

The contacts oi the minimum-voltage-responsive differential-relay 43 are utilized, in Fig. 5, in series with the paralleled contacts of the three current-differential relays 46a, 48b and Me to energize either one or the other of two contactorswitches 46 and 62, which are utilized to energize the trip-coils TC of all of the circuit breakers CB connected to whichever bus is the faulted bus, one contactor-switch 45 being provided for the bus- Bl breakers, while the other contactor-switch 4! controls the breakers connected to the bus B2.

In order to make sure that the differential-current relays 48a, 40b and 400 shall not have a faster operating-time than the minimum voltage differential-relay 43, I have illustrated the differential-current relays as being provided with timedelay means which are symbolically indicated by means of dashpots 48, which may be utilized as occasion demands.

In all of the embodiments of my invention, the minimum-voltage-responsive device may be any kind of voltage-responsive device, whether mechanical or tube-type, or whether polarized or non-polarized. A polarized relay is usually preferred, to a non-polarized relay, as it requires less energy-input, but my invention is not limited to a polarized relay, or to a. relay at all, as distinguished, for example, from a voltage-responsive device of any kind, such as the voltage-regulator shown in Fig. l, or in general, the resistance R shown in Fig. 2.

I wish it also to be understood that the general discussion given in connection with the network of Fig, 2 is applicable to all of the embodiments of the invention, so that the auxiliary voltagesource Es may be utilized, or omitted, or given any voltage-value desired, in connection with any of the figures of the drawing. In like manner, instead of having three variabl control-voltages Ea, Eb and EC, I might have only one variable voltage Ea, which is compared with one fixed voltage Es, so that the variable voltage Ea will control the relay-voltage Emn whenever E3, is less than Emn, but not otherwise. It is also possible, either with or without Es, to have two, or four, or any other number of control-voltages such as those illustrated at Ea, Eb and E0.

In Fig. 6, I have illustrated my invention applied to a tube-type undervoltage fault-detector relaying-system utilizing a gas-filled hot-cathode grid-glow tube 53 having an anode 5|, a grid 52, and a heated cathode 53, with the anode-cathode circuit connected in the tripping-circuit of the circuit-breaker 22, through the directional relay D or other discriminatory relay, as described in connection with Fig. 3. The grid 52 of the tube is connected to the negative terminal n of the minimum-voltage network 15', while the positive network-terminal m. is connected to the cathode 53 of the tube, so that the grid 52 is kept negative, with respect to the cathode 53, by a voltage which is controlled by the output-voltage Ernn of the network. The tube 50 has the characteristic of remaining non-conductive as long as the grid 52 is suificiently negative with respect to the cathode 53. When one of the three line-voltages drops to a sufliciently low value, the negative bias on the grid 52 will decrease to a value low enough to permit the grid to glow, thus establishing a tripping-circuit as soon as the other relay-contact D is closed, and thereafter the tube remains conducting, independentl of its gridpotential, until its plate-anode circuit is interrupted by some other means, such as the auxiliary breaker-switch 22a.

In the particular minimum-voltage network l5' shown in Fig. 6 I have not utilized an auxiliary source Es, but have utilized the back-currentflow of the seriall connected rectifiers l1 and I! of the high-voltage phases to supply the extremely small charging-current necessar to maintain the negative grid-voltage on the tube, it being understood that the grid-cathode circuit of the tube operates as an extremely high resistance R in the equivalent-circuit diagram of Fig. 2.

I wish it to be understood, however, that the auxiliary source Es could be added, in Fig. 6, or it could be omitted in any of the other figures where the operating-conditions warrant such omission, and also that a mechanical-type undervoltage relay could be utilized, in Fig. 6, in place of the tube 50, or a tube-type relay could be utilized in place of the mechanical-type voltage-responsive devices in any of the other figures,

It will be understood, of course, from Equation 17, that whenever the source Es and its serially connected resistance Rs is used, better operation may be obtained by making the relay-resistance R considerably less than the reverse-current rectifier-resistance NF, which may mean, in the case of a tube-relay, shunting the grid-cathode circuit with a loading-resistance in order to properly reduce the overall relay-resistance R.

The quick-acting voltage-regulator system which is shown in Fig. 1 is more particularly claimed in a divisional application Serial No. 512,443, filed December 1, 1943.

In connection with the broad idea of utilizing a minimum-voltage response to energize a volt-- age-responsive device so as to be responsive to the smallest phase of a polyphase electrical quantity, I wish my illustrations to be regarded as broadly indicative of any equivalent means to that end, as I believe that it is a new inventive concept to utilize a minimum-voltage response, in place of either a polyphase-energized torque-producing means or a polyphase-energized sequence-selective network, as a means for obtaining a response to a reduction in any one of the three line-voltages, regardless of which phase-voltage is the reduced voltage.

I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

I claim as my invention:

l. A voltage-responsive network comprising a pair of supply-voltage terminals, 9. pair of outputvoltage terminals, a pair of control-voltage terminals, circuit-means including impedance for joining th pair of output-voltage terminals to the pair of supply-voltage terminals for providing a rcsis'tively conducting current-path ior current 'from the pair" of supply-voltage terminals to the pair of output-voltage terminals, and an asymmetrically conductingcircuitmeans for joining "the pair of output-voltage terminals to the pair of control-voltageterminals fo'r pr'ovi'ding a good-conductor current-path 'for current flowing from 'the pair of output-voltage terminals to the pair o'fcontrol-voltage terminals but not "for current flowing in'the reverse direction, whereby, under operating conditions, when the voltage of the control-voltage terminals is lessthan tlievoltage oi the output-voltage terminals, current "flows fromthe'pair ofoutput-voitagefterminals to the pair of control-voltage terminals and pulls down the voltage of the output-voltage terminals to "a value approaching that of the control-voltageterminals.

2. Aminimum-volta'ge network comprising a plurality of pairs of control-voltageterminals, a pair of output voltage'terminalsand. a separate asymmetrically'conducting'circuit-means'forJoiningfthe pair ofoutput-voltageterminals to each oi theseveralpair of control-voltage terminals for providing a good-conductor current-path "for current flowing 'from the pair of output-voltage terminalstoits associated pair ofcon'trol-voltage terminals and for providing a resistively conducting'current path for current flowing from said associated pair of control-voltageterminalsto'the pair of output-voltage terminals, wherebyjunder operating conditions, current flows, at times, from the pair of output-voltage terminals to whichever pair ofcontrol-voltage terminals'has the lowest control-voltage applied thereto from some externally connected circuit, pulling down the'voltage s1 the'output voltageterminals toa value appreaching said lowest control-voltage.

3. A minimum-voltage network comprising a pair of supply-voltage'terminals,"a plurality of pairs of contrl=voltage terminals'a'pair 'of output-voltage terminals, circuit-means including resistance for joining the pair of output-voltage terminals tothe pair of supply-voltageterminals for providing a resistively conducting currentpath'for current flowing from'the pair of supplyvoltage terminals to the pair of output-voltage terminals, and a, separate asymmetrically conducting circuit means for joining the pair'of output-voltage terminals to each of'the severalpairs of control-voltage terminals for providing a goodconductor current-path for current flowingirom the pair of output-voltage terminaisto its associated pair of control-voltage terminais'but not for current flowing in the reverse directionflwhereby,'under operating conditions, current'flows, at times, from the pair of output-voltage terminals to whichever pair of control-voltage terminals has the lowest control-voltage applied thereto 'from some externally connected circuit, pulling down the voltage of the output-voltage terminals to a value approaching said" lowest control-voltage.

"4. i i voltage-network comprising "a source'of unidirectional current havingavoltageuvhich decreases with increasing current, a-voltage-respon- 'sive-device connected thereto, a'sou'rce of variable unidirectional control-voltage, and an "asymmetrically conducting circuit-means for connecting said control-voltage source to the terminals ofsaid voltage-responsive devicefor providing a good conductor current pathfor current flowing "from the terminals of the 'voltage-responsivedevice-to the control-voltage source but-not for currentflowing in thereversedirection.

'6. 'Avcltage-network comprising a plurality of sources of variable unidirectional control-voltages, a voltage-responsive I device, and a separate asymmetrically conducting circuit m'eans for connecting each of the control-voltage-sources to the voltage-responsive device in the same-polaritylfor providing a good-conductor-currentpath forcurrent flowing from the terminals of the voltagere'spon'si-Ve device "to'its associated-control voltase sourceand forpr'oviding a resistively'conducting current-path for current flowing from said associated control-voltage source to the voltage-responsive device.

6. A 'voltage network comprising a source of unidirectional current havinga-voltage which decreases' with increasing current, a voltage-responsive device connected thereto, a plurality of sources "of variable unidirectional control-voltages, "and a separate asymmetrically conducting circuit-means -forconnecting each of the controlvoltages'ources to the voltage-responsive device-in the same polarity -'as the first-mentioned source for providing a good-conductor current-path for current flowing from the terminals ofithe voltage-responsive device to its associated controlvoltagesource 'but'not 'for current flowing in the reversenflection.

7. rhe invention as definedinclaim a ber terized by said control-voltage source comprising a source of alternating current, a rectifier energized therefrom, anda 'resistance energized with unidirectional current from said'rectiifier, the terminals "of the resistance being connected to the asymmetrically conducting circuit-means, the network-constants being such that the current which is ifedirom the terminals ofzthe voltageresponsive devi'ce to the terminals of the resistance whichis also being energized :from the 'TCOII- trol-VoItagesource, whenthe latterisrlomis less thanthe energizing-current which is fed into said resistance from's'aid control-voltage source.

8. Theinvention as deflned inclaim 5 characterized by each of said control-voltage "sources comprising a source of alternating current, a rectifier energized therefrom, and a resistance energized with unidirectional current fromsaid rectifier, the terminals of the resistance being connected to the associated asymmetrically conducting circuit-means, the network-constants being such that-the current which is fed'from the terminals of the voltage-responsive device to the terminals of the resistance which is also being energized from a control-'voltagesource, when the latter. is low, is r less than the energizing-current Which-is fed into saidresistance from said control-voltage source.

9. The invention as defined in claim 6, characterized byeach of said control-voltagesources comprising a source of alternating current, a

rectifier energized therefrom, and a resistance energized with unidirectional current from said rectifien'the terminals of the resistance being connected to the associated asymmetrically conducting circuit-means, the network constants becontrol-voltage source.

10. A voltage-network comprising 'a storagebattery,':-a resistona voltage-responsive device, circuits-means 'for connecting the terminals -of "said voltage-responsive device tothe terminals of" said storage-battery through said resistor, a

source of unidirectional control-voltage which is normally higher than the battery-voltage but which is subject to occasional voltage-dips to voltages lower than the voltage across the terminals of said voltage-responsive device, a recti fier having a low resistance to current-flow in the forward direction and having a high resistance to back-current, and circuit-means for connecting the terminals of said control-voltage source, through aid rectifier, to the terminals of said voltage-responsive device in the same polarity as said battery, with the rectifier connected in such direction as to provide a low-resistance path for current flowing from the terminals of the voltageresponsive device to the terminals of the controlvoltage source, whereby the forward-currentbow through the rectifier pulls down the voltage of the voltage-responsive device to a value approaching the control-voltage when the latter is less than the former, and whereby the back-current-flow through the rectifier trickle-charges the battery when the control-source voltage is higher than the battery-voltage.

11. A voltage-network for discriminating as to which one of three or more sources of unidirectional voltages has the lowest voltage, comprising a pair of terminals for each of said sources, a pair of network-terminals, and a plurality of parallel-connected branch-circuits, one for each source, connected across said pair of network-terminals, each branch-circuit including, in series with its source-terminals, a. rectifier directed so as to preferentiall conduct current in a direction against the voltage of its source.

12. A voltage-network for discriminating as to which one of three or more sources of unidirectional voltages has the lowest voltage, comprising an electro-responsive device, and a plurality of parallel-connected branch-circuits, one for each source, connected across terminals of said electro-responsive device, each branch-circuit including, in series with its source, a rectifier directed so a to preferentially conduct current in a direction against the voltage of its source.

13. The invention as defined in claim 11, characterized by each source comprising a resistor connected in its branch-circuit, a supply-circuit connected across the resistor, a source of controlcurrent for said supply-circuit, and a rectifier in the supply-circuit for permitting current-flow in substantially only one direction.

14. The invention as defined in claim 12, characterized by each source comprising a resistor connected in its branch-circuit, a supply-circuit connected across the resistor, a source of controlcurrent for said supply-circuit, and a rectifier in the supply-circuit for permitting current-flow in substantially only one direction.

15. A lowest-quantity-responsive apparatus comprising the combination, with a polyphase line, of means for deriving, from said line, a plurality of polyphase electrical quantities of which the one which is lowest in magnitude is to be selected for response, a separate conversionmeans associated with each of the derived electrical quantities and having a pair of control-voltage terminals having a unidirectional voltage thereacross which is responsive to its electrical quantity, a unidirectional-current electro-responsive device, and a separate circuit-means including a serially connected rectifier for individually connecting each pair of control-voltage terminals to the terminals of said electro-responsive device in such manner as to freely draw current away from the terminals of the electro-responsive device to the lowest-voltage control-voltage terminals when the lowest control-voltage is lower than the voltage of the electro-responsive device, whereby, under such circumstances, the voltage of the electro-responsive device is brought down to a value approaching the lowest control-voltage.

16. The combination, with a polyphase line which is subject to single-phase faults, of a faultresponsive relay, means for deriving a plurality of diiferent line-current-responsive quantities, means for applying to said fault-responsive relay a relaying-current dependent upon the highest of said line-current-responsive quantities, means for deriving a. plurality of diiferent line-voltageresponsive quantities, and means for applying to said fault-responsive relay a relaying-voltage dependent upon the lowest of said line-voltageresponsive quantities.

17. The combination, with a polyphase line which is subject to single-phase faults, of a faultresponsive relay, a selective-phase-sequence current-responsive filter-means associated with said line for deriving a relaying-quantity which is responsive to any one of a plurality of different kinds of faults on different phases of the line, means for deriving, from the line, a relayingquantity which is responsive substantially to the lowest of the phase-voltages of the line, and means for utilizing these two relaying-quantiites in the energization of the relay whereby the relay selectively responds to the particular phase which is affected in a predetermined manner by the fault.

18. The combination, with a polyphase line which is subject to singlaphase faults, of a faultresponsive relay, means for actuating the relay in response to the polyphase line-current, no matter which phase is faulted, and means for restraining the relay in response to the lowest of the phase-voltages of the line, whereby the relay selectively responds to the particular phase which is affected in a predetermined manner by the fault.

19. The combination, with a polyphase electrical system, of means for deriving, from each of two different points in said system, a relaying quantity which is responsive substantially to the phase which is distinctively different, in magnitude, from the other phases of an electricallinequantity at that point, and relaying-means for comparatively responding to said two relaying quantities.

20. The combination, with a polyphase electrical system having two polyphase buses and polyphase connecting-means having sensible impedanoes for connecting said buses in parallel, of a differentially responsive protective relay for determining which of said two buses is faulted, in the event of a fault, and means for diiferentially energizing the relay in response to the lowest of the phase-voltages of the respective buses, whereby the relay selectively responds to the particular phase which is affected in a predetermined manner by the fault.

21. The combination, with a polyphase electrical system having circuit-interrupting sectionalizing-means for segregating a faulted portion of the system from the rest of the system, of a protective relaying-means for selectively energizing said sectionalizing-means in the event of a fault, an undervoltage relay, means for utilizing said undervoltage relay in controlling the operation of said protective relaying-means, and a minimumvoltage-responsive device, responsive to the system-voltage, for energizing said undervoltage relay in accordance with the lowest phase-voltage of the system, whereby, at times of a single-phase fault affecting any one of the phase-voltages of the system, the undervoltage relay will selectively respond to the particular phase which is the most affected by the fault.

22. A fault-responsive relay for selectively respending to a particular phase which is affected in a predetermined manner by a fault involving less than all the phases of a polyphase line to be protected, comprising means for deriving a current-responsive relaying-quantity dependent upon the line-current, means for deriving a set of polyphase voltage-responsive relaying-quan tities dependent upon the line-voltage, conversion-means comprising a plurality of sources of variable unidirectional voltage-responsive con trol-voltages responsive to different phases of said set of polyphase voltage-responsive relayingquantities, a voltage-network comprising a pair of minimum-voltage terminals, a separate asymmetrically conducting circuit-means for connecting each of the control-voltage sources to the minimum-voltage terminals in such manner as to freely draw current away from said minimumvoltage terminals to the lowest-voltage controlvoltage terminals when the lowest control-voltage is lower than the voltage of the minimumvoltage terminals, and means for utilizing the current-responsive relaying-quantity and the voltage across said minimum-voltage terminals in the energization of said fault-responsive relay.

23. The invention as defined in claim 22, characterized by a source of unidirectional current having a voltage which decreases with increasing current, and means for connecting said source to said minimum-voltage terminals in such polarity as to feed current to said lowest-voltage control-voltage terminals.

24. The invention as defined in claim 22, characterized by said means for deriving said currentresponsive relaying-quantity comprising a pair of maximum-voltage terminals for carrying said current-responsive relaying-quantity, and conversion-means and connecting-means for developing a plurality of variable unidirectional current-responsive control-voltages responsive to different phases of the polyphase line-current and for unidirectionally applying said currentresponsive control-voltages in parallel-circuit relation to said pair of maximum-voltage terminals in such manner as to feed current to said maximum-voltage terminals substantially only from the largest of said current-responsive contro1- voltages.

EDWIN L. HARDER. 

